The present application relates to a color conversion sheet enabling white light to be extracted using blue light as excitation light and to an illumination device and a display device using the same.
A liquid crystal display (LCD) is used as a thin-type display device. In the liquid crystal display, a backlight for illuminating an entire liquid crystal panel from the back side is used. According to the structure of the backlight, liquid crystal displays are broadly divided into the direct type and the edge light type. In the edge light type, light is allowed to enter from the side face of a light guide plate and propagate on the inside, thereby performing surface light emission on the top surface of the guide plate. In the direct type, surface light emission is performed by disposing a plurality of cold cathode fluorescent lamps (CCFLs) in the same plane. In displays at present, the direct type is the main stream (see Japanese Unexamined Patent Application Publication No. 2005-108635).
On the other hand, in recent years, attention is being paid to a backlight using light emitting diodes (LEDs) of three colors of red (R), blue (B), and green (G) aiming for enlargement of color gamut. In such a backlight, by disposing a plurality of light emitting diodes of three colors at predetermined intervals and simultaneously turning on them, white light is extracted by mixing the colors.
In the backlight using light emitting diodes of three colors, however, in the case of extracting uniform white color, a space for mixing the color light is necessary, so that the backlight becomes thicker by the space. To address the disadvantage, there is proposed a method of extracting white light by using only a light emitting diode of blue as a light source and combining a phosphor layer performing color conversion using blue light as excitation light with the blue light emitting diode (see, for example, Japanese Unexamined Patent Application Publication No. 2006-49657). According to the method, as compared with the case of using light emitting diodes of three colors, the backlight is made thinner.
As such a phosphor, for example, a phosphor obtained by adding a rare-earth material to a sulfide material is used. The phosphor, however, has a drawback such that it is sensitive to moisture vapor in atmosphere and deteriorates when exposed to moisture vapor. To address the drawback, there are proposed a method of hermetically sealing the inside of an exterior cap of a blue light emitting diode chip by applying and forming a phosphor layer on the inner face of the cap (Japanese Unexamined Patent Application Publication No. 2004-352928) and a method of sealing a phosphor layer by sandwiching it by two glass substrates (Japanese Unexamined Patent Application Publication No. 2006-163939).